1. Field
The present disclosure generally relates to induction heating techniques, and deals more particularly with a method and device for inductively heating relatively large areas using multiple induction heating coils with smart susceptors powered at relatively low voltages.
2. Background
Induction heating may be used in a wide variety of industrial processes to elevate the temperature of parts or structures. For example, in the field of composites, induction heating may be used to cure one or more portions of a structure formed from composite materials such as fiber reinforced polymer resins. In order to achieve thermal uniformity during the heating process, an induction heating system may use induction coils magnetically coupled with susceptors to translate electrical power into heat energy. The susceptors are sometimes referred to as “smart” susceptors because the materials from which they are formed are specifically chosen to produce a maximum, constant temperature when inductively heated. This equilibrium constant temperature is achieved at the Curie point of the susceptor material. The Curie Point is the temperature at which there is a transition between the ferromagnetic and nonmagnetic phases of the material. Once the Curie temperature is reached, the susceptors become non-magnetic and greatly reduce their heating rate. This built-in thermostatic control provides a means of avoiding overheating and allows precise temperature control. The susceptors may be in various physical forms, including but not limited to sleeves or spiral warps placed around induction coils, plates, or magnetic particles dispersed within a surrounding matrix.
Induction heating systems using smart susceptors of the type described above may be used, for example, as heating blankets to cure small areas of a composite, such as composite patches used to rework of an aircraft skin. However, this induction heating technique may not be practical where relatively large areas of a part or a structure need to be heated. For example, in order to cover larger part areas, multiple induction heating coils may be coupled together in series. Coupling the coils in series over large areas may have disadvantages in some applications, including the need to use relatively high drive voltages because of the cumulative resistance the coils present to the electrical power source. Also, series coupled coils may be more difficult to control, presenting the possibility of thermal runaway and/or uneven heating of the part.
Accordingly, there is a need for an induction heating device that provides uniform, controlled heating of large surface areas, using multiple self-balancing induction coil circuits. There is also a need for an induction heating method and device that uses relatively low voltages to drive multiple induction coils with smart susceptors covering relatively large surface areas.